Vanderbilt's Bobby Johnson has abruptly retired as the Commodores' head coach.

Vice chancellor David Williams announced the news in a post on the university's athletic website Wednesday just before a news conference.

An emotional Johnson called it a very difficult decision but one not prompted by health concerns for either himself or his wife. And he clarified that he was not merely stepping down from a job, but walking away from coaching.

"I've decided to retire, not resign," Johnson said. "It's a tough decision with which my wife, Catherine, and I struggled. This is a personal decision."

"Football is not life, but it's a way of life and it consumes your life," Johnson said. "You only have so many years to live, and you want to see a different way."

Williams announced that Robbie Caldwell will be the interim head coach after serving as assistant head coach.

"I am saddened by the fact Bobby has made this decision," Williams said. "At the same token, as I've said to the staff, it's our obligation to respect his decision. I did try to talk Bobby out of it. I tried to talk him out of it yesterday. I tried to talk him out of it last night, and I tried to talk him out of it this morning."

Johnson's resignation comes a week before he was scheduled to appear at the Southeastern Conference's preseason media days, nearly three weeks before players report for fall practice and just seven weeks before the Commodores' opener Sept. 4 against Northwestern.

Bobby Johnson led Vanderbilt to its second-ever bowl game win in 2008. Al Messerschmidt/Getty Images

"This is quite a shock to us," Caldwell said.

Coaching college football is demanding, and Johnson indicated he wasn't ready to continue giving 100 percent.

"If you don't have the investment in a season where it means everything to you, then I don't think you should be coaching," Johnson said.

Johnson, who turned 59 earlier this year, is tied for the fourth-most wins in the history of the program, although the Commodores are coming off their first winless season in the SEC since Johnson's first season at Vanderbilt in 2002.

The Commodores had experienced a resurgence under Johnson and won their first bowl game in 53 years, defeating Boston College in the 2008 Music City Bowl. Johnson was named SEC Coach of the Year that season.

Johnson's career record at Vanderbilt is 29-66.

"This isn't something you can sit around and mope about," defensive lineman T.J. Greenstone said. "Having Coach Caldwell helps a lot. He knows the kind of people we have here. He knows how the system works. He knows the personnel. We're not planning on missing a beat here."

Defensive lineman Adam Smotherman, a Tennessee native, credited Johnson with changing Vanderbilt from a favorite homecoming opponent to a program with a chance to win every game.

"I hope he is happy and finds I guess the happiness that he's looking for out of life. We're going to move on. We are Vanderbilt football. We're going to keep going and keep trucking," Smotherman said.

Williams said he realized Johnson was serious when he called the coach's house late Tuesday night and spoke with Catherine Johnson.

"I could tell from her voice I had lost," Williams said.

Caldwell will be given the chance to keep the job with his performance this season. Williams said he has a coach right now and has no plans for a national search.

"I'm not searching for anything other than a victory and a lot of them," Williams said.

Caldwell has never been a head coach before outside of a stint as a high school baseball coach for one season in South Carolina. He didn't even have time to alert his family with the news and had to quickly change from shorts to pants for the news conference.

As Caldwell answered questions about whether he was ready to be head coach, Johnson interrupted.

"He's ready," Johnson said with a smile.

Chicago Bears quarterback Jay Cutler, who played for Johnson at Vanderbilt, said he was shocked by the coach's retirement.

"It was something I'm sure Coach Johnson has been thinking about for a long time. I don't think he woke up yesterday morning and decided to hang it up," he said in an interview Thursday with ESPN Radio's "Mike and Mike in The Morning".

"It's a big loss, I think, for Vanderbilt, but luckily enough they kept the staff intact. I think it's one of the best staffs in the SEC. Those guys fight an uphill battle, but they get after it each and every week and do a great job. I'm happy to see they kept those guys around for another year for another chance," he said in the interview.

Johnson was hired as Vanderbilt's coach in 2001, after leading Furman University to the Football Championship Subdivision (Division I-AA) national championship game.

In 2008, Johnson produced one of the sport's greatest turnarounds, leading the Commodores to a 5-0 start and a national ranking. Vanderbilt upset then-No. 24 South Carolina 24-17 and defeated No. 14 Auburn 14-13, both at home, to climb as high as No. 13 in the national polls.

The Commodores lost six of their last seven regular-season games, before beating Boston College 16-14 in the Gaylord Music City Bowl in Nashville -- the second bowl victory in school history. It was Vanderbilt's first nonlosing season since 1982.

Johnson also ended losing skids to eight other SEC teams during his tenure, including a 22-game string to Tennessee in 2005.

But he lost the top recruit of his incoming class in February when running back Rajaan Bennett of Georgia, the highest-rated player ever signed to Vanderbilt, was shot to death at his home.

Two of Johnson's former SEC coaching colleagues said Johnson's teams at Vanderbilt made the most of their ability.

"They were very disciplined and played to their strengths," said Mississippi State coach Dan Mullen, a former offensive coordinator at Florida. "They played hard. He got the most out of his players."

Current Mississippi coach Houston Nutt, who previously coached at Arkansas, said Johnson is "an excellent coach who did an awesome job with less."

Chris Low is a college football writer for ESPN.com. Information from ESPN.com's Mark Schlabach, ESPN's Joe Schad and The Associated Press contributed to this report.